Project Rendering in Black and White?
Hi @TobiasPenner and welcome. When I opened your Blend file I noticed that the two plains have a white emission material and the cube has the default light-gray. So when you say you viewed as colour I am not sure what you mean. You may have changed a few things before submitting the file, but maybe if you showed an image of the different modes it will help.

Anyhow with your blend file if you want to add color to the cube and see in Material preview mode you will need to do the following:


Change your cube to view as a solid object (it is displayed as a wireframe). 

Assign a color in the material properties of the object